NAME

SYNOPSIS

DESCRIPTION

The write() function shall attempt to write nbyte bytes
from the buffer pointed to by buf to the file
associated with the open file descriptor, fildes.

Before any action described below is taken, and if nbyte is
zero and the file is a regular file, the write()
function may detect and return errors as described below. In the absence
of errors, or if error detection is not performed, the
write() function shall return zero and have no other results.
If nbyte is zero and the file is not a regular file,
the results are unspecified.

On a regular file or other file capable of seeking, the actual writing
of data shall proceed from the position in the file
indicated by the file offset associated with fildes. Before
successful return from write(), the file offset shall be
incremented by the number of bytes actually written. On a regular
file, if this incremented file offset is greater than the length
of the file, the length of the file shall be set to this file offset.

On a file not capable of seeking, writing shall always take place
starting at the current position. The value of a file offset
associated with such a device is undefined.

If the O_APPEND flag of the file status flags is set, the file offset
shall be set to the end of the file prior to each write
and no intervening file modification operation shall occur between
changing the file offset and the write operation.

If a write() requests that more bytes be written than there
is room for (for example, the process'
file size limit or the physical end of a medium), only as many
bytes as there is room for shall be written. For example, suppose
there is space for 20 bytes more in a file before reaching a
limit. A write of 512 bytes will return 20. The next write of a non-zero
number of bytes would give a failure return (except as
noted below).

If the request would cause the file size to exceed the soft file size
limit for the process and there is no room for any bytes to
be written, the request shall fail and the implementation shall generate
the SIGXFSZ signal for the thread.

If write() is interrupted by a signal before it writes any data,
it shall return -1 with errno set to [EINTR].

If write() is interrupted by a signal after it successfully
writes some data, it shall return the number of bytes
written.

If the value of nbyte is greater than {SSIZE_MAX}, the result
is implementation-defined.

After a write() to a regular file has successfully returned:

*

Any successful read() from each byte position in the file that
was modified by that
write shall return the data specified by the write() for that
position until such byte positions are again modified.

*

Any subsequent successful write() to the same byte position
in the file shall overwrite that file data.

Write requests to a pipe or FIFO shall be handled in the same way
as a regular file with the following exceptions:

*

There is no file offset associated with a pipe, hence each write request
shall append to the end of the pipe.

*

Write requests of {PIPE_BUF} bytes or less shall not be interleaved
with data from other processes doing writes on the same
pipe. Writes of greater than {PIPE_BUF} bytes may have data interleaved,
on arbitrary boundaries, with writes by other processes,
whether or not the O_NONBLOCK flag of the file status flags is set.

*

If the O_NONBLOCK flag is clear, a write request may cause the thread
to block, but on normal completion it shall return
nbyte.

*

If the O_NONBLOCK flag is set, write() requests shall be handled
differently, in the following ways:

*

The write() function shall not block the thread.

*

A write request for {PIPE_BUF} or fewer bytes shall have the following
effect: if there is sufficient space available in the
pipe, write() shall transfer all the data and return the number
of bytes requested. Otherwise, write() shall transfer
no data and return -1 with errno set to [EAGAIN].

*

A write request for more than {PIPE_BUF} bytes shall cause one of
the following:

*

When at least one byte can be written, transfer what it can and return
the number of bytes written. When all data previously
written to the pipe is read, it shall transfer at least {PIPE_BUF}
bytes.

*

When no data can be written, transfer no data, and return -1 with
errno set to [EAGAIN].

When attempting to write to a file descriptor (other than a pipe or
FIFO) that supports non-blocking writes and cannot accept
the data immediately:

*

If the O_NONBLOCK flag is clear, write() shall block the calling
thread until the data can be accepted.

*

If the O_NONBLOCK flag is set, write() shall not block the thread.
If some data can be written without blocking the
thread, write() shall write what it can and return the number
of bytes written. Otherwise, it shall return -1 and set
errno to [EAGAIN].

Upon successful completion, where nbyte is greater than 0, write()
shall mark for update the st_ctime and
st_mtime fields of the file, and if the file is a regular file,
the S_ISUID and S_ISGID bits of the file mode may be
cleared.

For regular files, no data transfer shall occur past the offset maximum
established in the open file description associated with
fildes.

If fildes refers to a socket, write() shall be equivalent
to send()
with no flags set.

If the O_DSYNC bit has been set, write I/O operations on the file
descriptor shall complete as defined by synchronized I/O data
integrity completion.

If the O_SYNC bit has been set, write I/O operations on the file descriptor
shall complete as defined by synchronized I/O file
integrity completion.

If fildes refers to a shared memory object, the result of the
write() function is unspecified.

If fildes refers to a typed memory object, the result of the
write() function is unspecified.

If fildes refers to a STREAM, the operation of write()
shall be determined by the values of the minimum and maximum
nbyte range (packet size) accepted by the STREAM. These values
are determined by the topmost STREAM module. If nbyte
falls within the packet size range, nbyte bytes shall be written.
If nbyte does not fall within the range and the
minimum packet size value is 0, write() shall break the buffer
into maximum packet size segments prior to sending the data
downstream (the last segment may contain less than the maximum packet
size). If nbyte does not fall within the range and the
minimum value is non-zero, write() shall fail with errno
set to [ERANGE]. Writing a zero-length buffer ( nbyte
is 0) to a STREAMS device sends 0 bytes with 0 returned. However,
writing a zero-length buffer to a STREAMS-based pipe or FIFO
sends no message and 0 is returned. The process may issue I_SWROPT
ioctl() to enable
zero-length messages to be sent across the pipe or FIFO.

When writing to a STREAM, data messages are created with a priority
band of 0. When writing to a STREAM that is not a pipe or
FIFO:

*

If O_NONBLOCK is clear, and the STREAM cannot accept data (the STREAM
write queue is full due to internal flow control
conditions), write() shall block until data can be accepted.

*

If O_NONBLOCK is set and the STREAM cannot accept data, write()
shall return -1 and set errno to [EAGAIN].

*

If O_NONBLOCK is set and part of the buffer has been written while
a condition in which the STREAM cannot accept additional data
occurs, write() shall terminate and return the number of bytes
written.

In addition, write() shall fail if the STREAM head has processed
an asynchronous error before the call. In this case, the
value of errno does not reflect the result of write(),
but reflects the prior error.

The pwrite() function shall be equivalent to write(),
except that it writes into a given position without changing
the file pointer. The first three arguments to pwrite() are
the same as write() with the addition of a fourth
argument offset for the desired position inside the file.

RETURN VALUE

Upon successful completion, write() and pwrite()
shall return the number of bytes actually written to the file associated
with fildes. This number
shall never be greater than nbyte. Otherwise, -1 shall be returned
and errno set to indicate the error.

ERRORS

The write() and pwrite() functions shall fail
if:

EAGAIN

The O_NONBLOCK flag is set for the file descriptor and the thread
would be delayed in the write() operation.

EBADF

The fildes argument is not a valid file descriptor open for
writing.

EFBIG

An attempt was made to write a file that exceeds the implementation-defined
maximum file size or the
process' file size limit, and there was no room for any bytes
to be written.

EFBIG

The file is a regular file, nbyte is greater than 0, and the
starting position is greater than or equal to the offset
maximum established in the open file description associated with fildes.

EINTR

The write operation was terminated due to the receipt of a signal,
and no data was transferred.

EIO

The process is a member of a background process group attempting to
write to its controlling terminal, TOSTOP is set, the
process is neither ignoring nor blocking SIGTTOU, and the process
group of the process is orphaned. This error may also be returned
under implementation-defined conditions.

ENOSPC

There was no free space remaining on the device containing the file.

EPIPE

An attempt is made to write to a pipe or FIFO that is not open for
reading by any process, or that only has one end open. A
SIGPIPE signal shall also be sent to the thread.

ERANGE

The transfer request size was outside the range supported by the STREAMS
file associated with fildes.

The write() function shall fail if:

EAGAIN or EWOULDBLOCK

The file descriptor is for a socket, is marked O_NONBLOCK, and write
would block.

ECONNRESET

A write was attempted on a socket that is not connected.

EPIPE

A write was attempted on a socket that is shut down for writing, or
is no longer connected. In the latter case, if the socket
is of type SOCK_STREAM, the SIGPIPE signal is generated to the calling
process.

The write() and pwrite() functions may fail
if:

EINVAL

The STREAM or multiplexer referenced by fildes is linked (directly
or indirectly) downstream from a multiplexer.

EIO

A physical I/O error has occurred.

ENOBUFS

Insufficient resources were available in the system to perform the
operation.

ENXIO

A request was made of a nonexistent device, or the request was outside
the capabilities of the device.

ENXIO

A hangup occurred on the STREAM being written to.

A
write to a STREAMS file may fail if an error message has been received
at the STREAM head. In this case, errno is set to the
value included in the error message.

The write() function may fail if:

EACCES

A write was attempted on a socket and the calling process does not
have appropriate privileges.

ENETDOWN

A write was attempted on a socket and the local network interface
used to reach the destination is down.

ENETUNREACH

A write was attempted on a socket and no route to the network is present.

APPLICATION USAGE

None.

RATIONALE

See also the RATIONALE section in read() .

An attempt to write to a pipe or FIFO has several major characteristics:

*

Atomic/non-atomic: A write is atomic if the whole amount written
in one operation is not interleaved with data from any
other process. This is useful when there are multiple writers sending
data to a single reader. Applications need to know how large
a write request can be expected to be performed atomically. This maximum
is called {PIPE_BUF}. This volume of
IEEE Std 1003.1-2001 does not say whether write requests for more
than {PIPE_BUF} bytes are atomic, but requires that
writes of {PIPE_BUF} or fewer bytes shall be atomic.

*

Blocking/immediate: Blocking is only possible with O_NONBLOCK
clear. If there is enough space for all the data requested
to be written immediately, the implementation should do so. Otherwise,
the process may block; that is, pause until enough space is
available for writing. The effective size of a pipe or FIFO (the maximum
amount that can be written in one operation without
blocking) may vary dynamically, depending on the implementation, so
it is not possible to specify a fixed value for it.

This shall never happen if nbyte<= {PIPE_BUF}. If it does happen
(with nbyte> {PIPE_BUF}), this volume of
IEEE Std 1003.1-2001 does not guarantee atomicity, even if ret<=
{PIPE_BUF}, because atomicity is guaranteed
according to the amount requested, not the amount written.

Deferred:

ret=-1, errno=[EAGAIN]

This error indicates that a later request may succeed. It does not
indicate that it shall succeed, even if
nbyte<= {PIPE_BUF}, because if no process reads from the pipe
or FIFO, the write never succeeds. An application could
usefully count the number of times [EAGAIN] is caused by a particular
value of nbyte> {PIPE_BUF} and perhaps do later
writes with a smaller value, on the assumption that the effective
size of the pipe may have decreased.

Partial and deferred writes are only possible with O_NONBLOCK set.

The relations of these properties are shown in the following tables:

Write to a Pipe or FIFO with O_NONBLOCK clear

Immediately Writable:

None

Some

nbyte

nbyte<={PIPE_BUF}

Atomic blocking

Atomic blocking

Atomic immediate

nbyte

nbyte

nbyte

nbyte>{PIPE_BUF}

Blocking nbyte

Blocking nbyte

Blocking nbyte

If the O_NONBLOCK flag is clear, a write request shall block if the
amount writable immediately is less than that requested. If
the flag is set (by fcntl()), a write request shall never block.

Write to a Pipe or FIFO with O_NONBLOCK set

Immediately Writable:

None

Some

nbyte

nbyte<={PIPE_BUF}

-1, [EAGAIN]

-1, [EAGAIN]

Atomic nbyte

nbyte>{PIPE_BUF}

-1, [EAGAIN]

<nbyte or -1,

<=nbyte or -1,

[EAGAIN]

[EAGAIN]

There is no exception regarding partial writes when O_NONBLOCK is
set. With the exception of writing to an empty pipe, this
volume of IEEE Std 1003.1-2001 does not specify exactly when a partial
write is performed since that would require
specifying internal details of the implementation. Every application
should be prepared to handle partial writes when O_NONBLOCK is
set and the requested amount is greater than {PIPE_BUF}, just as every
application should be prepared to handle partial writes on
other kinds of file descriptors.

The intent of forcing writing at least one byte if any can be written
is to assure that each write makes progress if there is
any room in the pipe. If the pipe is empty, {PIPE_BUF} bytes must
be written; if not, at least some progress must have been
made.

Where this volume of IEEE Std 1003.1-2001 requires -1 to be returned
and errno set to [EAGAIN], most historical
implementations return zero (with the O_NDELAY flag set, which is
the historical predecessor of O_NONBLOCK, but is not itself in
this volume of IEEE Std 1003.1-2001). The error indications in this
volume of IEEE Std 1003.1-2001 were chosen
so that an application can distinguish these cases from end-of-file.
While write() cannot receive an indication of
end-of-file, read() can, and the two functions have similar
return values. Also, some
existing systems (for example, Eighth Edition) permit a write of zero
bytes to mean that the reader should get an end-of-file
indication; for those systems, a return value of zero from write()
indicates a successful write of an end-of-file
indication.

Implementations are allowed, but not required, to perform error checking
for write() requests of zero bytes.

The concept of a {PIPE_MAX} limit (indicating the maximum number of
bytes that can be written to a pipe in a single operation)
was considered, but rejected, because this concept would unnecessarily
limit application writing.

See also the discussion of O_NONBLOCK in read() .

Writes can be serialized with respect to other reads and writes. If
a read() of file
data can be proven (by any means) to occur after a write() of
the data, it must reflect that write(), even if the
calls are made by different processes. A similar requirement applies
to multiple write operations to the same file position. This
is needed to guarantee the propagation of data from write()
calls to subsequent read() calls. This requirement is particularly
significant for networked file systems, where
some caching schemes violate these semantics.

Note that this is specified in terms of read() and write().
The XSI
extensions readv() and writev() also
obey these semantics. A new "high-performance" write analog that did
not follow these serialization requirements would also be
permitted by this wording. This volume of IEEE Std 1003.1-2001 is
also silent about any effects of application-level
caching (such as that done by stdio).

This volume of IEEE Std 1003.1-2001 does not specify the value of
the file offset after an error is returned; there
are too many cases. For programming errors, such as [EBADF], the concept
is meaningless since no file is involved. For errors that
are detected immediately, such as [EAGAIN], clearly the pointer should
not change. After an interrupt or hardware error, however,
an updated value would be very useful and is the behavior of many
implementations.

This volume of IEEE Std 1003.1-2001 does not specify behavior of
concurrent writes to a file from multiple processes.
Applications should use some form of concurrency control.

COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .